A Primer on Binocular Rivalry,

Including Current Controversies

Randolph Blake

Vanderbilt Vision Research Center

Vanderbilt University

Nashville TN 37240

USA

running head: Binocular Rivalry

email: randolph.blake@vanderbilt.edu

voice: 615-343-7010

FAX: 615-343-5029

Binocular Rivalry

Abstract.

Among psychologists and vision scientists, binocular rivalry has enjoyed sustained interest for decades dating back

to the 19th century. In recent years, however, rivalry’s audience has expanded to include neuroscientists who

envision rivalry as a “tool” for exploring the neural concomitants of conscious visual awareness and perceptual

organization. For rivalry’s potential to be realized, workers using this “tool” need to know details of this

fascinating phenomenon, and providing those details is the purpose of this article. After placing rivalry in a

historical context, I summarize major findings concerning the spatial characteristics and the temporal dynamics of

rivalry, discuss two major theoretical accounts of rivalry (“eye” vs “stimulus” rivalry) and speculate on possible

neural concomitants of binocular rivalry.

key words: binocular rivalry, suppression, conscious awareness, neural model, perceptual organization

1. Introduction

The human brain has been touted as the most complex structure in the known universe (Thompson, 1985).

This may be true, but despite its awesome powers the brain can behave like a confused adolescent when it is

confronted with conflicting visual messages. When dissimilar visual stimuli are imaged on corresponding retinal

regions of the two eyes, the brain lapses into an unstable state characterized by alternating periods of perceptual

dominance during which one visual stimulus or the other is seen at a time. This confusion is understandable, for

the eyes are signalling the brain that two different objects exist at the same location in space at the same time. Faced

with this physical impossibility, the brain, metaphorically speaking, entertains two alternative perceptual

interpretations, changing its mind repeatedly. This fluctuation in perception, of course, is the phenomenon termed

binocular rivalry, the focus of this special issue of Brain and Mind. Actually, in the early years after its

discovery, the phenomenon sometimes was termed “retinal rivalry” in recognition of the fact that the two retinae

were the source of the discordant binocular input. During the last century, that term was supplanted by “binocular

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Binocular Rivalry

rivalry.” And very recently, some have advocated the even more neutral term “stimulus rivalry” because

alternations in perceptual dominance can arise even when two competing stimuli are not unequivocally associated

with the two eyes.

In this article I have four goals: a) to provide a very brief historical account of binocular rivalry, b) to

summarize major findings concerning the spatial characteristics and the temporal dynamics of rivalry, c) to discuss

two major theoretical accounts of rivalry (“eye” vs “stimulus” rivalry) and the evidence bearing on those

accounts, and d) to speculate on possible neural concomitants of binocular rivalry. Several highly relevant topics

are not included in this article, since they are covered by other contributors to this special issue of Brain and Mind;

those timely topics include brain imaging studies during rivalry, hemispheric specialization and rivalry, and [add

more here].

From the outset, I want to forewarn readers that the literature on binocular rivalry is quite large, and a

comprehensive review is beyond the scope of this article. I also wish to acknowledge the availability of several

excellent reviews of work on binocular rivalry, including publications by Fox (1991), by Logothetis (1998), by

Howard and Rogers (1995) and by Papathomas et al (1999). Some of the content in this article echoes material

covered in those reviews, and students of binocular rivalry will gain a balanced overview of the field by consulting

those sources. As well, those interested in rivalry will find it worthwhile to capitalize on the very thorough, up-to-

date reference list of papers on binocular rivalry maintained by Dr. Robert O’Shea -- that list may be accessed on

the world-wide web at:

http://psy.otago.ac.nz/r_oshea/br_bibliography.html

Throughout this article, I will be describing aspects of rivalry that readers may want to experience first-hand.

Toward that end, I have prepared a webpage with demonstrations that can be viewed using red/green filter glasses;

the URL for that webpage is:

http://www.psy.vanderbilt.edu/faculty/blake/rivalry/BR.html

The webpage also lists several good commercial sources for purchasing effective, inexpensive anaglyphic glasses.

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Binocular Rivalry

2. Background

Surveying the historical literature on vision and optics, one can identify a number of quotations indicating an

appreciation of the perceptual consequence of discordant binocular stimulation. Indeed, the noted vision scholar

Nicholas Wade, in his delightful book on A Natural History of Vision (Wade, 1998), describes no fewer than eight

accounts of what can be construed as binocular rivalry. According to Wade, it is Porta (1593) who deserves credit

for the first unambiguous description of rivalry. This description was based on Porta’s observation upon viewing

separately with the two eyes different pages from two books. (Porta accomplished this simply by placing a

partition between the two eyes, so that the left eye could only see the page on the left side of the partition and the

right eye only the page before the right eye.) According to Porta, only one page was visible and, hence, available to

consciousness for reading; according to Porta, it was the right eye, for him, that dominated in perception. So

Porta’s descriptions clearly capture one essential ingredient of binocular rivalry: the suppression from vision of

one eye’s view.

Following Porta, a number of vision scholars commented on the occurrence of rivalry, and some went so far

as to invent more elaborate optical means (e.g., prisms) to promote the stimulus conditions for rivalry. However, it

is Sir Charles Wheatstone (1838) who deserves credit for the first systematic study of rivalry, which can be found

in his famous monograph on binocular stereopsis. Using his newly invented mirror stereoscope (Figure 1a),

Wheatstone noted an odd breakdown in normal binocular cooperation when the two eyes separately viewed

different alphabetic letters. To quote Wheatstone:

“If a and b [shown as S and A in Figure 1b] are each presented at the same time to a different eye, the

common border will remain constant, while the letter within it will change alternately from that which

would be perceived by the right eye alone to that which would be perceived by the left eye alone. At the

moment of change the letter which has just been seen breaks into fragments, while fragments of the

letter which is about to appear mingle with them, and are immediately after replaced by the entire letter.

It does not appear to be in the power of the will to determine the appearance of either of the letters, but

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the duration of the appearance seems to depend on causes which are under our control: thus if the two

pictures be equally illuminated, the alternations appear in general of equal duration; but if one picture be

more illuminated than the other, that which is less so will be perceived during a shorter time.”

Insert Figure 1 about here

So in this short passage Wheatstone offered several trenchant observations on key aspects of rivalry,

including the complete suppression of one of two discordant stimuli, the alternations in dominance between the

eyes, the spatial fragmentation of the two images during times of transition, and the dependence of predominance

on the physical characteristics of the rival stimuli. These concise observations have been replicated and extended in

subsequent work on rivalry.

Following Wheatstone’s seminal essay, binocular rivalry captured the imagination of some of science’s

leading figures, including von Helmholtz, William James and Sir Charles Sherrington, and during the 20th century

literally hundreds of papers were published on the topic. Earlier work on rivalry has been reviewed by Levelt

(1965) and by Walker (1978), and more contemporary work is reviewed by Fox (1991) and by Howard and

Rogers (1995). For purposes of relating rivalry to possible underlying neural mechanisms, it is useful to begin by

summarizing a number of well supported generalizations about rivalry.

3. Characteristics of Binocular Rivalry

These next several subsections summarize well-established characteristics of rivalry, with particular emphasis

on factors that influence the spatial and temporal dynamics of rivalry.

3.1 What stimulus differences trigger rivalry?

Left- and right-eye differences along any one of a wide range of stimulus dimensions are sufficient to

instigate binocular rivalry. These include differences in color, luminance, contrast polarity, form, size and motion

4

Fig. 1. a) A reproduction of the drawing of Sir Charles Wheatstone’s newlyinvented mirror stereoscope, taken from his monograph on binocular vision(Figure 8 in Wheatstone, 1838). An observer would situate his head so that theright eye would see the image reflected in the right-hand mirror (A’ in the figure)and the left eye would see the image reflected in the left-hand mirror (A). Eachmirror would reflect whatever picture Wheatstone placed in the holders (E’ and E)attached to the upright boards (D’ and D) situated at opposite ends of the arms ofthe stereoscope. The arms are attached to two boards (C’ and C) that could bemoved different distances from the mirrors. b) A drawing reproduced fromWheatstone’s monograph (Figure 25) showing a pair of dissimilar “half-images”that inspired Wheatstone rather detailed descriptions of binocular rivalry.

S AFig. 1b) A drawing reproduced from Wheatstone’s monograph (Figure 25)showing a pair of dissimilar “half-images” that inspired Wheatstone rather detaileddescriptions of binocular rivalry.

Binocular Rivalry

velocity. Rivalry can be triggered by very simple stimulus differences (e.g., gratings differing only in orientation)

and by complex differences (e.g., pictures of a human face viewed by one eye and a house viewed by the other).

Rivalry can be observed over a very wide range of light levels, including scotopic, and it can be observed anywhere

throughout the binocular visual field so long as the stimuli are adjusted for visibility. There appear to be only a few

conditions of dichoptic stimulation (i.e., different monocular stimuli imaged on corresponding retinal areas) that

resist rivalry; these include dichoptic differences in flicker rate (O’Shea and Blake, 1986), dichoptic differences in

contrast level (to be distinguished from contrast polarity), large differences in left- and right-eye spatial frequency

amplitude spectra (Yang et al, 1992) and large differences in speed of random-dot motion (van de Grind et al, in

press). Liu et al (1992) have reported that many seconds may be required before one experiences rivalry when both

dichoptically viewed, orthogonally oriented gratings are near-threshold in contrast, particularly high spatial

frequency gratings.

Alternations in perceptual dominance -- the hallmark of binocular rivalry -- can also be produced under some

conditions of monocular viewing (Breese, 1899; Campbell and Howell, 1972), and, of course, there are many well-

known ambiguous figures that produce fluctuations in perceptual interpretation (e.g., the vase/face figure). Some

researchers believe that all these instances of perceptual fluctuation -- binocular rivalry, monocular rivalry,

ambiguous figures -- result from common mechanisms (e.g., Andrews and Purves, 1997), an idea that deserves

careful consideration. The present review will be confined to studies of patent binocular rivalry, where dissimilar

stimuli are presented to corresponding regions of the two eyes.

3.2 Spatial extent of rivalry.

In the literature, one often reads that binocular rivalry entails alternating periods of monocular dominance and

suppression between discordant views seen by the left and right eyes. Particularly for large rival stimuli, however,

this characterization is not entirely correct. Rather than alternating between periods of exclusive dominance of one

eye’s view and then the other eye’s view, one often experiences a fluctuating patchwork consisting of intermingled

portions of both eyes’ views (Meenes, 1930); the incidence of “patchwork” rivalry increases with prolonged

viewing of rival targets (Hollins and Hudnell, 1980). Sometimes dubbed “piecemeal” rivalry, these periods of

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mixed dominance suggest that rivalry transpires locally within spatially restricted “zones” and not globally

between the eyes. How large are these putative zones? For foveally viewed rival stimuli, it has been estimated that

exclusive, unitary rivalry (no periods of piecemeal rivalry) can be obtained only when rival stimuli do not exceed

about 0.1 deg visual angle. However, this critical angular subtense increases with retinal eccentricity (Blake et al

1992) and with spatial frequency (O’Shea et al 1997), and the incidence of piecemeal rivalry is much reduced when

rival targets are viewed at low light levels (O’Shea et al 1994). Figure 2a presents several pairs of rival figures,

varying in their angular subtense. Readers capable of free-fusion can compare the incidence of piece-meal rivalry

among these different pairs of figures. Also included in this Figure is a pair of rival targets (Figure 2b)

demonstrating that suppression spreads some distance beyond the immediate regions of binocular conflict, again

pointing to the possible existence of zones of suppression (Kaufman, 1963; Collins and Blackwell, 1974).

Insert Figure 2 about here

There is reason to believe that the spatial extent of rivalry is governed by the retinal angular subtense of the

competing stimuli, not by the perceived size of those stimuli (Blake et al, 1974). Specifically, when rival afterimages

of small angular subtense are viewed against a surface located far from the observer, the rival targets appear quite

large thanks to Emmert’s law (perceived size of an object associated with a fixed size retinal image increases with

perceived distance: Palmer 1999). However, those apparently “large” afterimages in fact tend to rival in an all or

none fashion in accordance with their small angular subtense. Conversely, rival afterimages of large angular

subtense exhibit piecemeal dominance even when those afterimages are viewed against a very nearby surface which

renders their apparent size quite tiny. Now, it is generally thought that variations in the perceived size of an

afterimage of constant angular subtense reflect the operation of neural mechanisms that scale size in terms of

distance. On this account, then, the spatial extent of rivalry must be determined by neural events uninfluenced by

this putative size-scaling mechanism.

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Fig. 2. The three pairs of rival targets on the left allow readers to experience howtarget size influences the incidence of piecemeal rivalry (i.e., rivalry in whichdifferent portions of targets seen by the two eyes can be dominant in perceptionsimultaneously). Only with the smaller pair of rival targets do observersconsistently experience exclusive dominance of one entire target or the other, withrelatively few moments of piecemeal rivalry.

left eye right eye

X X

Fig.2b. This pair of rival targets allows readers to see how suppression spreadsbeyond the immediate location of conflict. After fusing the two half-images, fixatethe x-mark and notice how the vertical contour often disappears in its entirety,even though the middle portion has no competing partner in the other eye.

Binocular Rivalry

One intriguing, suggestive characteristic of rivalry concerns the spatial appearance during transitions in

dominance between one stimulus and the other. Rather than being haphazard, these transitions often appear as

spatially coherent waves of dominance wherein one stimulus appears to sweep the other from conscious awareness.

This observation could be construed to imply that the putative “zones” of rivalry are not independent, which leads

to the following question. Are fluctuations in rivalry within one portion of the visual field unrelated to rivalry

fluctuations within other portions of the visual field? If local suppression zones were indeed completely

independent, we would expect a relatively large stimulus viewed by one eye (i.e. one subtending many degrees of

visual angle) to be visible, or dominant, in its entirety only very rarely. However, complete monocular dominance

does occur significantly more often than would be expected based on chance alone (Blake et al 1992). In a similar

vein, multiple small rival targets scattered throughout the visual field can engage in synchronized alternations, such

that all targets of a given configuration (e.g. all the green circles) are dominant simultaneously (Whittle et al, 1968;

Dörrenhaus, 1975; Kovács et al 1996; Alais and Blake, 1999). This observation, too, implies that rivalry zones are

not independent. Finally, it has been found that visual features located outside the boundaries of a rival target can

influence the predominance of that rival target, as though the strength of the rival target were being modulated by its

neighbors (Ichihara and Goryo, 1978; Mapperson and Lovegrove, 1991; Fukuda and Blake 1992; Alais and Blake,

1998). This finding, likewise, points to spatial interactions beyond the boundaries of any putative local zones of

rivalry.

3.3 Temporal dynamics of rivalry

Binocular rivalry is not experienced when dissimilar monocular stimuli are very briefly exposed -- instead,

the perceptual result with brief exposure resembles the binocular superimposition of the two stimuli (Anderson et

al, 1978; Wolfe, 1983; Blake et al, 1991). So, for example, a pair of briefly flashed, orthogonally oriented rival

gratings looks like a plaid.1 To experience complete dominance of one rival figure over another requires that both

rival figures be presented simultaneously for at least several hundred milliseconds. Moreover, rivalry is disrupted if

1.Nor does a pair of rival targets “pop out” from an array of otherwise binocularly fused targets (Wolfe,1988), implying that it takes time for the visual system to register monocularly incompatible stimuli.

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Binocular Rivalry

dissimilar monocular stimuli are flickered on-and-off rapidly and repetitively (O’Shea and Crassini, 1984). These

observations suggest that rivalry is highly susceptible to transient stimulation. In fact, it is well known that, during

rivalry, a phenomenally suppressed stimulus can be readily restored to dominance by manoeuvres that create

transient stimulation within the eye viewing that stimulus. Thus, for example, simply waving a pencil in front of a

suppressed stimulus immediately brings it into dominance, erasing the other eye’s view from consciousness

(Grindley and Townsend, 1965); the same effect can be achieved by an abrupt increase in the contrast of a

suppressed stimulus (Blake and Fox, 1974a) or by setting a suppressed stimulus into motion (Fox and Check,

1968; Walker, 1975; Walker and Powell, 1979).

What transpires when rival targets are viewed for periods lasting many seconds, without perturbation? In this

case, individual periods of dominance and suppression are unpredictable in duration, being sequentially

independent random variables (Levelt, 1965; Fox and Herrmann 1967; Blake et al, 1971; Wade, 1975; Walker,

1975; Lehky 1995). Individual dominance durations conform reasonably well to the gamma distribution, an

example of which is shown in Figure 3. Comparable frequency distributions are found when monkeys experience

binocular rivalry (Myerson et al, 1981; Logothetis, 1998), a noteworthy finding that validates the use of non-human

primates for the study of rivalry.

Aside from deploying oculomotor “tricks”2 observers seem unable willfully to trigger immediate switches

from suppression to dominance or to hold one stimulus dominant indefinitely (Blake, 1988).3 There is quite a bit

of evidence, however, that attentional manipulations can bias overall predominance in favor of one of two rival

figures during extended tracking periods. Helmholtz (1925) observed that he could bias dominance in favor of one

2. One can very easily terminate suppression of a stimulus by blinking the eye viewing the suppressedstimulus or by moving the eyes in a way that differentially creates transients from that stimulus (e.g.,horizontal eye movements while a vertical grating is suppressed).

3. Lack (1978) speculates that people might develop the ability to control rivalry completely if givensufficient practice. His speculation was stimulated by the observation that people can selectively controlwhich of two dissimilar messages are heard during dichotic listening; selective listening, Lack points out, isa skill at which we’re all well practiced.

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rival target by concentrating his attention on that target using strategies like counting the number of lines in a given

rival stimulus. Even then, however, Helmholtz found that attention’s efficacy seemed to wane, for a dominant

stimulus eventually gives way to suppression involuntarily after a period of time. Helmholtz’s observations have

since been documented in a number of studies showing that people can learn to influence the alternation rate of

rivalry (e.g., Lack, 1969). The role of attention in determining rivalry predominance has been very systematically

studied by Lack (1978) who, among other things, showed that control exerted over rivalry is not attributable solely

to the operation of peripheral mechanisms such as blinking or accommodation. Instead, central neural processes

are implicated. In addition to documenting that attention can promote dominance, Ooi and He (1999) showed that

directing attention to the region of an eye where a rival target is currently suppressed can prematurely terminate

suppression of that target.

Besides enhancing predominance and protecting a stimulus from suppression, attention to one of two rival

targets may also promote exclusive dominance (i.e., minimize piecemeal rivalry) when those targets subtend a large

angular subtense (Neisser and Becklen, 1975), an idea proposed by Ooi and He (1999). When local rivalry

components are pieced together into a globally coherent, dominant image -- a phenomenon discussed in the

previous section (Kovács et al, 1996) -- perhaps attention is providing part of the neural glue. As we continue to get

a clearer picture of how the responses of visual neurons are modulated by attention, including neurons at the very

earliest cortical stages of processing, we will be in a better position to theorize about attention’s modulatory effect

on rivalry predominance and appearance.

Insert Figure 3 about here

The most straightforward means for influencing predominance is to vary some aspect of the competing

stimuli. A large body of literature confirms that manipulation of stimulus variables such as luminance (Kakizaki,

1960; Fox and Rasche 1969), contrast (Whittle, 1965; Hollins, 1980), contour density (Levelt 1965), spatial

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Fig. 3. Histogram summarizing the durations of individual periods of dominanceduring binocular rivalry. Observers “tracked” dominance of two dissimilar patternsby pressing one of two keys, and the individual durations were tabulated,normalized (each value was divided by the grand mean) and plotted as a frequencyhistogram. The solid line shows the gamma function fit to the actual data. Theskewness of this function varies with rivalry alternation rate, becoming morenearly gaussian when that rate is rapid (Myerson et al, 1981). (Graph adapted withpermission from Figure 2 of Fox, R. and Herrmann, J. 1967: Stochastic propertiesof binocular rivalry alternations, Percept. Psychophys. 2, 432-436.)

Binocular Rivalry

frequency (Hollins, 1980; Fahle, 1982; Andrews and Purves, 1997), size (O’Shea et al, 1997), velocity (Breese

1899; Wade et al, 1984; Blake et al, 1998) and retinal eccentricity (Fahle, 1987), to name just a few, can produce

pronounced variations in alternation rate and in stimulus predominance. As a generalization, a “stronger” rival

target (e.g., one that is higher contrast than the other) enjoys enhanced predominance, defined as the total

percentage of time that a given stimulus is visible during an extended viewing period. Moreover, there is evidence

that this increased predominance is produced, in large part, by decreases in the durations of suppression periods of

a given stimulus - “stronger” stimuli tend to stay suppressed for shorter periods of time while not necessarily

staying dominant for longer periods of time (e.g., Levelt, 1965; Fox and Rasche 1969; but see Mueller and Blake,

1989, and Bossink et al, 1993, for evidence pointing to longer dominance durations, too). If both pairs of rival

targets are bilaterally increased in strength (e.g., the contrasts of both are high), each target remains suppressed for

more abbreviated periods of time and, consequently, the two targets alternate in dominance more rapidly (e.g.,

Levelt, 1965). Rivalry alternations measured with a given stimulus configuration slow markedly in older

individuals, and this slowing of rivalry is only partially attributable to reduced retinal illumination (Jalavisto, 1964).

It is generally believed that transitions from dominance to suppression are mediated by neural adaptation,

with the excitatory signals from the dominant stimulus weakening over time and eventually succumbing to the

signals from the other, previously suppressed stimulus (e.g., Sugie, 1982; Lehky, 1988; Mueller, 1990). Consistent

with this idea, a stimulus forced to remain dominant during rivalry (by incrementing its contrast the moment it

becomes suppressed) generates increasingly brief dominance durations, as if being denied an opportunity to

recover from adaptation (Blake et al, 1990); once dominance is no longer forced, that stimulus remains suppressed

for an unusually long period of time.

There is also some evidence purporting to show an influence of cognitive and motivational factors on

stimulus predominance (see Walker, 1978 for a review of this evidence). In these kinds of studies, the two eyes

view dissimilar monocular stimuli that vary in their emotional content or their meaningfulness to an observer.

According to some studies, a more cognitively salient stimulus exhibits greater predominance during rivalry. For

example, when Jewish and Catholic observers were asked to judge the relative predominance of symbols associated

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with their two religions (e.g., the star of David versus a cross) Jewish religious symbols tended to predominate for

the Jewish observers while Catholic religious symbols tended to predominate for the Catholic observers (LoSciuto

and Hartley, 1963). In a similar vein, upright pictures of a human face tend to predominate over inverted pictures of

a face (Engel, 1956), and figures one has seen before tend to predominate over novel figures (Goryo, 1969). These

and comparable results have been interpreted to mean that non-visual factors such as affective content influence the

resolution of stimulus conflict during binocular rivalry (Walker, 1978). Such an influence would be very important

in shaping our search for possible neural concomitants of binocular rivalry. Moreover, such an influence is not far-

fetched given the evidence for attention’s modulatory role in rivalry (Lack, 1978). There are, however, reasons to

interpret these findings cautiously (Yu and Blake, 1992). Because they measured predominance using self-report,

the measures of dominance were susceptible to demand characteristics. Moreover, these studies often used rival

targets that were large and, therefore, prone to piecemeal rivalry, yet the judgements were binary in nature;

observers had to adopt some criterion for declaring stimulus predominance, thus compounding the problem of

response bias. It will be useful to see these kinds of experiments repeated using procedures that minimize these

potential problems and, where possible, rely on more objective measures of rivalry dominance, such as the test

probe method described in the following section.

3.4 What is suppressed during binocular rivalry?

When a given stimulus is suppressed during rivalry, the neural mechanisms underlying suppression perforce

must be operating on the features composing that stimulus. But to what extent does suppression generalize to other

stimulus features? The answer to this question depends on how it is framed experimentally. For one class of tasks

(test probe tasks), suppression appears to be quite general, encompassing a wide range of stimulus conditions. For

a second class of tasks (co-existence tasks), however, suppression appears more restricted. The following

subsections briefly review results from these two categories of tasks, along with their implications.

3.4.1 Test probe sensitivity Suppression periods of rivalry are accompanied by a general loss in visual sensitivity.

Thus, a probe target superimposed on a stimulus engaged in rivalry is more difficult to detect when that probe is

presented while the rival target is suppressed, compared to detection performance when the same probe is presented

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Binocular Rivalry

during rival target dominance (Wales and Fox, 1970; Fukuda, 1981; O’Shea and Crassini, 1981; Smith et al,

1982). Moreover, it is not just psychophysically measured visual sensitivity that is impaired during suppression --

a light stimulus presented to a suppressed eye evokes less pupillary constriction than does that same light

presented to the eye during dominance (Barany and Hallden, 1948; Lorber et al, 1965; Brenner et al, 1969).

Impaired detection performance is found even when probes differ markedly from the suppressed target itself.

Thus, for example, a test probe consisting of one of several possible alphabetic characters is difficult to recognize

when briefly superimposed on a simple, uniform disk that was suppressed during rivalry (Fox and Check, 1972).

In a similar vein, reaction times to highly visible probes are lengthened when those probes are presented to an eye

during suppression (Fox and Check, 1968; O’Shea, 1987). This depression in visual sensitivity is also evidenced

by an inability to detect changes in a suppressed rival target, so long as those changes are not accompanied by

abrupt transients in luminance or contrast. Thus, for example, observers fail to detect large changes in the spatial

frequency or the orientation of a suppressed grating (Blake and Fox 1974a), and they fail to detect transitions from

incoherent, random motion to coherent, structured motion (Blake et al 1998). Even when changes in orientation are

accompanied by abrupt transients, reaction times for detection of those changes are lengthened (O’Shea and

Crassini, 1981).

This nonselective loss in visual sensitivity represents a hallmark of suppression, and it has been construed to

imply that suppression is operating not just on the stimulus features of the initially suppressed target but rather on

the region of the eye upon which that target is imaged (Blake, 1989). It is important to keep in mind, however, that

this region of nonselective suppression is limited in spatial extent and does not encompass an entire eye. Indeed,

this is why piecemeal rivalry is often observed with large targets: different regions of an eye can be in different

states of rivalry. It is also noteworthy that this general reduction in test-probe performance during suppression

corresponds to only about 0.5 log-unit loss in sensitivity, relative to that measured during dominance (e.g.,

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Binocular Rivalry

Holopigian, 1989).4 This seems like a very modest sensitivity loss, compared to the wholesale loss in the visibility

of the rival target itself. After all, a complex, high contrast image several orders of magnitude above detection

threshold can disappear entirely for several seconds -- here suppression seems to be depressing visibility by

several log-units. However, a 0.5 log-unit range of stimulus energy (e.g., contrast) is sufficient under some

conditions to span an entire psychometric function, ranging from chance to perfect performance. Viewed in this

light, a 0.5 log-unit change in sensitivity could be sufficient to move a stimulus from reliably detectable to reliably

undetectable. In any event, we presently have no idea why probes are difficult to detect during suppression, and we

would like an answer to that question in order to know what the results from probe experiments are telling us about

rivalry suppression.

3.4.2 Co-existence tasks. What aspects of visual perception manage to co-exist with binocular rivalry, unperturbed

by fluctuations in dominance and suppression? Answers to this question can have important implications for the

issue of the selectivity of suppression (Blake, 1997). The following paragraphs highlight findings that bear on

those answers.

3.4.2.1 Stereopsis. It is natural to wonder how stereopsis and rivalry relate to one another, since both phenomena

involve binocular interactions between disparate monocular views. In the case of stereopsis, those disparities

comprise slight perspective differences between the two eyes’ views, disparities that can be resolved by synthesis

of a stable, three-dimensional representation incorporating both views. In the case of rivalry, those disparities entail

dissimilar monocular features incompatible with the existence of a single, three-dimensional view; the consequence,

then, is a breakdown in stable, stereoscopic vision. It is interesting to note that dissimilar monocular stimulation

does not inevitably yield rivalry -- if dissimilar monocular stimulation is compatible with the pattern of stimulation

one would experience while viewing partial occlusion of one object by another, binocular vision tends to be stable

(Shimojo and Nakayama, 1990).

4. It is worth noting that probe sensitivity during dominance is equivalent to monocular sensitivitymeasured under non-rivalry conditions, implying that there is nothing unusual about the dominance state ofrivalry.

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Binocular Rivalry

But what happens when stereo half-images are presented simultaneously with rival half-images? Can

stereopsis co-exist in the face on ongoing rivalry? The evidence for coexistence is mixed. Several studies have

reported that observers experience patent stereopsis even when the pair of stereo half-images are superimposed on

dissimilar monocular figures undergoing vigorous binocular rivalry (Treisman, 1962; Ogle and Wakefield, 1967;

Julesz and Miller, 1975; Mayhew and Frisby, 1976; Blake et al, 1979). Several other studies, however, report the

opposite: stereopsis is disrupted when stereo half-images are viewed during rivalry (Hochberg, 1964; Amira,

1989). In an attempt to reconcile these seemingly conflicting results, Harrad et al (1994) measured stereo

thresholds under conditions where one stereo half-image was presented continuously to one eye and a rival target

was presented to the other eye. When the stereo half-image was suppressed in rivalry (as indicated by the

observer’s button press), the other stereo half-image was briefly superimposed on the other eye’s dominant rival

target (see Figure 4 for an example of one of these displays). By varying the exposure duration of the flashed half-

image, Harrad et al discovered that stereopsis was indeed impaired for the 150 milliseconds of exposure but, for

durations longer than this, stereo performance was equivalent to that measured under nonrivalry conditions. This

critical duration is the same order of magnitude as the threshold values reported by Julesz and Tyler (1976) for

detecting transitions from rivalry stimulation to fusion stimulation. It appears, then, that stereopsis can defeat

rivalry, after losing the initial, early stage of the battle.

Insert Figure 4 about here

3.4.2.2 Visual adaptation. Can a visual stimulus induce an adaptation aftereffect when that stimulus is suppressed

from vision for a substantial portion of the adaptation period? Several research groups have pursued this question,

examining a variety of well-known visual aftereffects. With two exceptions, suppression has no effect on

adaptation induction, meaning that a full-blown aftereffect is measured immediately following a period of

adaptation during which the adapting stimulus is rendered intermittently invisible owing to binocular rivalry.

14

Fig. 4. One of the several stimulus conditions used by Harrad et al to study theinteraction between stereopsis and binocular rivalry. Observers initially viewed thetop pair of half-images, which produced rivalry between the pair of offset verticallines seen by the left eye and the set of diagonal lines seen by the right eye. Whenthe offset lines were suppressed in their entirety, observers depressed a key whichtriggered the brief presentation of the pair of offset vertical lines superimposed onthe dominant diagonal lines in the right eye. This pair of offset lines, when pairedwith those imaged in the left eye, created the stimulus conditions for stereopsis(one line appears nearer than the other, in virtue of the relative disparities of thetwo). During blocks of trials the exposure duration of the briefly presented half-image was varied according to a method of constant stimuli, and durationthresholds for successful stereo performance were derived. For exposure durationless than 150 msec, thresholds measured during rivalry were elevated relative tothresholds measured under nonrivalry conditions. (Figure reproduced withpermission from Harrad, R.A., McKee, S.P., Blake, R. and Yang, Y., 1994:Binocular rivalry disrupts stereopsis, Perception.23, 15-28. Copyright Pion Press.)

Binocular Rivalry

Aftereffects (AE) that escape the sweep of suppression include the grating threshold elevation AE and the spatial

frequency shift AE (Blake and Fox, 1974b), the translational motion AE (Lehmkuhle and Fox, 1975; O’Shea and

Crassini, 1981) and the tilt AE (Wade and Wenderoth, 1978). The two exceptions to this rule are the spiral AE

(Wiesenfelder and Blake, 1990) and the spatial-phase AE associated with the square-wave illusion (Blake and

Bravo, 1985). This pattern of results has been interpreted to reflect the existence of multiple, hierarchically arranged

processing stages with rivalry suppression sandwiched between certain clusters of stages (Blake, 1995). Whether

or not this interpretation proves correct, these kinds of studies demonstrate that a stimulus is not necessarily

rendered completely impotent when suppressed from conscious awareness during rivalry.

3.4.2.3 Perceptual priming Prior exposure to a stimulus can subsequently facilitate judgements to other stimuli that

are visually or semantically similar to the originally experienced stimulus. Called “priming,” this facilitation can

take the form of improved recognition, faster picture naming or speeded discrimination of words from nonwords.

Two experiments performed in my laboratory have shown that a suppressed stimulus loses its potency as a

priming source (Zimba and Blake, 1983; Cave et al, 1998). To paraphrase the title from one of these articles (Zimba

and Blake, 1983), a visual pattern out of sight is also a visual pattern out of mind.

This conclusion does not hold for motion priming, however. A clever experiment by Anstis and

Ramachandran (1987) showed that people viewing an ambiguous apparent motion (AM) sequence experience one

of two possible patterns of motion with equal likelihood when the lengths of the alternative motion paths are

equivalent. Motion perception in this case is bistable. People can be biased, however, to see a given pattern of

motion if the ambiguous AM sequence is preceded by “priming” motion consistent with that pattern of motion.

Capitalizing on motion bistability, Blake, Ahlstrom and Alais (1998) found that motion perception could be biased

even when the priming motion was presented within the confines of a suppressed rival target, thus rendering the

priming motion completely invisible. This finding, incidentally, squares with earlier work showing that AM can be

perceived even when the initial frame of the AM sequence comprises a suppressed rival target (Wiesenfelder and

Blake, 1991), and work showing that motion signals from a suppressed eye can interact with those from the

dominant eye to determine perceived direction of motion (Carney et al, 1987; Andrews and Blakemore, 1999).

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Binocular Rivalry

4. What Rivals during Rivalry?

Descriptively speaking, binocular rivalry involves a battle for dominance between conflicting patterns, with the

temporary loser suppressed from consciousness for several seconds at a time. Moreover, the observer “sees” that

pattern without any knowledge of which eye is dominant at any given moment during rivalry - the observer “sees”

a stimulus without regard to its eye of origin. Based on appearances, then, it is entirely reasonable to believe that

rivalry involves competition between alternative perceptual interpretations -- indeed, this view has strong historical

precedence (see sidebar). Several lines of psychophysical evidence, however, imply that this belief may be incorrect

and that, instead, it is a region of one “eye” that is temporarily dominant, not a given “stimulus.” Consider the

eye swapping procedure illustrated schematically in Figure 5.

Insert Figure 5 about here

An observer depresses a switch when the rival pattern shown to one eye is dominant exclusively, the other

being completely suppressed from vision. If this keypress causes the two rival patterns to be swapped between the

two eyes, the previously dominant pattern will now be shown to the previously suppressed eye and vice versa.

When this happens, the outcome is clear (Blake et al 1980): the dominant pattern abruptly becomes invisible and

the previously suppressed pattern becomes dominant, implying that it was the region of an eye that was dominant,

not a particular stimulus. Control trials confirm that this outcome is not due simply to turning the patterns “off”

and then back “on.”

Next, consider the stimulus change procedure. Here the orientation, the spatial frequency or the direction of

motion of a stimulus is changed while that stimulus is suppressed. These changes, regardless how large, go

unnoticed until seconds later when the suppressed eye returns to dominance - again, it seems not to be a particular

pattern that is suppressed, but instead the region of an eye where a rival target is imaged (Blake and Fox 1974a;

Blake et al 1998). For “change” to go undetected during suppression, it is necessary to avoid sharp transients

16

caption on next page

Fig. 5. (a) Schematic of the eye swapping procedure deployed by Blake et al(1980). At the start of each trial, observers dichoptically viewed a pair oforthogonally oriented gratings, shown here as horizontal and vertical. Observersdepressed a key when the grating of one orientation was visible exclusively, withno hint of the other orientation. On “reversal” trials (R), button depression causedthe two gratings to be interchanged between the eyes; to eliminate abrupttransients, the contrasts of the two gratings were ramped off and then back on overa 150 msec period, as indicated by the inset in the Figure. On “non-reversal” trials(NR), button depression caused the gratings to be ramped off and back on, but theywere not exchanged between the eyes. On “ordinary” trials, the gratings remainedpresent continuously when observers depressed a button to declare a gratingdominant. Observers were instructed to release the button as soon as thesuppressed grating achieved dominance. If a given pattern were dominant inrivalry, moving that pattern to the other eye should have no effect on its durationof dominance, which normally lasts at least several seconds. But if a region of agiven eye is dominant during rivalry, swapping the orientations should produce animmediate transition in dominance, for the previously suppressed pattern wouldnow be imaged in the currently dominant eye.

Fig. 5 (b) Results showing that dominance durations during exchange trials (R) aresignificantly shorter than dominance durations during nonreversal trials (NR) andduring ordinary trials where nothing happened at all (N). This pattern of resultsindicates that the visibility of a stimulus during rivalry depends on which eye isviewing that stimulus. (Figure 5b is reproduced with permission from Blake, R.,Westendorf, D, and Overton, R., 1979: What is suppressed during binocularrivalry? Perception 9, 223-231. Copyright Pion Press.)

Binocular Rivalry

associated with replacement of one stimulus by another; as noted above, transients reliably break suppression

(Walker and Powell 1979).

A third piece of evidence pointing to a role for the “eyes” in rivalry comes from a study using stereo half-

images that included regions in which corresponding retinal areas viewed dissimilar patterns (Shimojo and

Nakayama, 1990). When that dichoptic stimulation was consistent with the partial occlusion (an ecologically valid

situation), binocular vision was stable, but when dichoptic stimulation was inconsistent with partial occlusion

binocular rivalry was experienced. In effect, these two alternative perceptual outcomes were dependent on which

eye received which pattern of dichoptic stimulation, indicating that the rivalry process takes into account the eye of

origin of monocular stimulation. A similar conclusion was reached by Ooi and He (1999), who found that a

monocularly viewed “pop-out” array could bias dominance and could weaken suppression of a given eye during

binocular rivalry; their results, too, require that the rivalry process have some record of which stimulus is being

viewed by which eye.

On the other hand, several other pieces of evidence favor a “stimulus” based theory of rivalry. For example,

rival targets differing in form and color can sometimes achieve states of dominance in which the color from one

eye’s image combines in dominance with the form from the other eye’s image, temporarily creating a binocular

impression that corresponds with neither monocular component (e.g., Breese 1909; Creed, 1935; but see Hollins

and Leung, 1978, for negative evidence on this point). These unusual periods of interocular combination could be

construed as illusory conjunctions of the sort observed with briefly flashed arrays of colored objects (Treisman

and Schmidt, 1982). Similarly, perceived direction of motion of a dominant stimulus can be influenced by the

velocity of a suppressed stimulus (Andrews and Blakemore, 1999). In both of these instances, the perceptual

experience during dominance phases of rivalry sometimes comprise an amalgam of attributes from both eyes’

views, which should be impossible if all information from a given region of one eye were lost during suppression.

Advocates of “stimulus” rivalry also point to “interocular grouping” during rivalry as evidence against

“eye” rivalry (Kovács et al 1996). Figure 6 illustrates one particularly captivating instance of this phenomenon.

With some practice viewing these patchwork rival figures, observers are able to see either of two globally coherent

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Binocular Rivalry

figures: the face of an ape or a complete array of text. These periods of coherent perception, of course, can only be

achieved by very specific combinations of left-eye and right-eye components that are dominant simultaneously;

obviously, one eye alone is not responsible for dominance. Evidence for simpler versions of interocular grouping

have been reported by others as well (Diaz-Caneja 1928, translated by Alais, O'Shea, Mesana-Alais, & Wilson,

2000; Dörrenhaus, 1975; Alais and Blake 1999).

Thus vision seems to be exploiting structural regularity to link distributed features in these kinds of displays,

and one might be tempted to attribute these linkages to “top-down” processes signifying meaningful perceptual

interpretations. This attribution, however, may be premature. Within early visual cortical areas, including V1, there

exist long-range, lateral connections among orientation-selective neurons, with these connections arranged so as to

strengthen responses among neurons signalling extended contours (Bosking et al, 1997; Das and Gilbert, 1995).

These lateral connections provide a possible neural substrate for embodiment of visual context effects, without the

need for “top-down” feedback (Field et al, 1993). Of course, there also is abundant evidence for feedback

connections from higher cortical areas to lower ones. Thus, it stands to reason that feedback information registered

during dominance phases of rivalry could impact neural events transpiring anywhere along the visual pathways, in a

manner surmised by Papathomas et al (1999). It remains for future work to reveal the extent to which visual image

meaning per se can promote dominance during rivalry. In the meantime, these compelling interocular grouping

phenomena certainly underscore that rivalry does not involve dominance of an entire eye; instead, spatially

distributed image features can be cooperatively grouped to promote figural coherence. But interocular grouping

does not rule out the possibility that the suppressed portions of an image are, in fact, linked to a given eye. Global

dominance may be achieved by the spatial amalgamation of “eye” based features.

Insert Figure 6 about here

Perhaps the most influential piece of evidence favoring “stimulus” rivalry is provided by a paper published

18

Fig. 6. Rival targets used by Kovacs et al (1996). Shown in the upper part of thefigure (A) is a pair of rival targets consisting of dissimilar pictures (in this case theface of an ape and a page of text) viewed by left and right eyes. Shown in thelower part of the figure (B) is a pair of rival targets each consisting of portions ofthe ape’s face and portions of the text. Upon viewing these mixed rival targets,observers occasionally experience periods during which the ape or the text isdominant completely, an outcome that can only be achieved by the combination ofimage fragments from both eyes. Kovacs et al found the incidence of completedominance to be greater than that expected on the basis of chance alone. Readersmay free-fuse these rival targets to judge the incidence of interocular grouping.(Figure reproduced with permission from Kovacs, I., Papathomas, T.V., Yang, M.and Fehér, A. 1997: When the brain changes its mind, Interocular grouping duringbinocular rivalry, Proc. Natl. Acad. Sci. USA 93, 15508-15511. Copyright NationalAcademy Press.)

Binocular Rivalry

in Nature by Logothetis et al (1996) in which they created novel stimulus conditions that unequivocally preclude

eye rivalry. Using a variant of the eye interchange technique, they repetitively exchanged the two eyes’ patterns

every 333 msec. In addition, both rival targets (low contrast, orthogonally oriented gratings) were rapidly flickered

at 18 Hz (see right-hand drawing in Figure 7, labelled “Exchange trial”). Observers viewing this unique display

experienced periods during which one grating was dominant for several seconds at a time, with dominance

fluctuating between the two gratings in a slow, irregular time-course just like that experienced during conventional

rivalry. Logothetis et al referred to this outcome as “stimulus rivalry” because observers were perceiving a given

stimulus for durations that exceeded the rate at which that stimulus was exchanged between the eyes -- clearly,

these extended periods of stimulus dominance could not be attributable to “eye” rivalry. Generalizing from their

results, Logothetis et al concluded that rivalry under conventional conditions, wherein stimuli are not swapped

between the eyes, also entails alternations in dominance between competing stimuli, not between competing eyes.

According to Logothetis (1998), “the dominance and suppression of a pattern during rivalry reflects the excitation

and inhibition of cell populations in the higher visual areas, which are directly involved in the representation of

visual patterns.” He echoes this view in a later essay (Logothetis 1999) by saying, “rivalry occurs because

alternate stimulus representations compete in the visual pathway (p. 73).”

Insert Figure 7 about here

Intrigued by Logothetis et al’s report and their conclusions, members of my laboratory felt it important to test

the generality of these findings. Accordingly, Sang-Hun Lee and I created modified versions of the unique displays

used by Logothetis et al, including ones that examined “eye” vs “stimulus” rivalry over a wider range of spatial

and temporal conditions (Lee and Blake, 1999). Specifically, we repetitively interchanged left and right eye rival

targets while, at the same time, flickering those targets at 18 Hz (“exchange trials” in Figure 7). We also

intermixed conventional, “non-exchange” trials on which rival targets were flickered at 18 Hz but were not rapidly

swapped between the eyes (“Non-exchange trials” in Figure 7). On any given trial observers did not know which

19

Fig. 7. Schematic of stimulus displays used by Logothetis et al (1996) and,subsequently, by Lee and Blake (1999). On some observation trials (exchangetrial), orthogonally oriented rival gratings are flickered at 18 Hz and areintermittently exchanged between the eyes at a brisk rate (shown here as 3 Hz, thevalue used by Logothetis et al); on other trials (non-exchange trial), the rivalgratings flicker at 18 Hz but are not exchanged between eyes. (Figure reproducedwith permission from Lee, S.H. and Blake, R., 1998: Rival ideas about binocularrivalry, Vision Res. 39, 1447-1454. Copyright Elsevier Press.)

Binocular Rivalry

type of presentation was being presented -- eye swapping or not -- and following each trial observers indicated

whether they experienced “slow, irregular” changes in dominance, “rapid, regular changes” in dominance, or

“mixed and patchy” dominance. All four observers, including three who were naive about the purpose of the

experiment, experienced “slow, irregular” changes in dominance under the conditions tested by Logothetis et al,

i.e., low contrast, high spatial frequency gratings interchanged between the eyes at 3 Hz. At slower exchange rates

and at lower spatial frequencies, however, these observers experienced “rapid, regular” fluctuations in dominance,

indicating that they were perceiving the regular flips in orientation presented to a given eye (i.e., “eye” rivalry). At

the lowest spatial frequencies tested, observers primarily experienced piecemeal rivalry. In other words, “stimulus”

rivalry is confined to a rather narrow range of spatial and temporal conditions (see Figure 8). We also found

that the incidence of “stimulus rivalry” within this restricted, optimal region of spatio-temporal space was

markedly dependent on the nature of the 18 Hz flicker. We picked the best spatial frequency and the best eye

exchange rate for producing “stimulus rivalry” and then tested using different conditions of temporal modulation.

When the rapid, 18 Hz flicker was eliminated, the incidence of “stimulus rivalry” dropped from 68% to 30%.

Moreover, when the non-flickering gratings were exchanged at 3 Hz between the eyes in a manner that eliminated

abrupt transients (gaussian contrast modulation rather than square-wave modulation), the incidence of “stimulus

rivalry” dropped to 10%.

Insert Figure 8 about here

Based on the results from this study, we (Lee and Blake, 1999) concluded that the narrow range of conditions

yielding “stimulus rivalry” effectively overrides normal rivalry, perhaps because of the complex train of transient

events accompanying rapid flicker and eye exchange. After all, it is known from the study by O’Shea and Crassini

(1984) that transients can disturb rivalry, and the same conclusion is implied by test probe experiments showing

that abrupt stimulus changes disrupt the normal course of rivalry (Fox and Check, 1968; Walker and Powell, 1984;

20

Fig. 8. These three graphs show the relative incidence of “stimulus” rivalry,conventional rivalry and piecemeal rivalry for different conditions of dichopticstimulation measured using the exchange technique illustrated in Figure 5. Forthese measurements, the two eyes viewed orthogonally oriented gratings thatflickered on/off at 18 Hz and were interchanged between the eyes at a rate thatvaried over blocks of trials (duration shown along the abscissa). Grating contrastwas constant at 25% and spatial frequency of both gratings varied over the rangeshown on the ordinate. Evidence for “stimulus” rivalry (slow, irregular change)was observed almost exclusively at high spatial frequencies and at fast rates of eyeexchange; throughout most of the spatio-temporal space, observers experiencedeither conventional rivalry (fast, regular change) or piecemeal rivalry (mixed,patchy grating). (Figure reproduced with permission from Lee, S.H. and Blake, R.,1998: Rival ideas about binocular rivalry, Vision Res. 39, 1447-1454. CopyrightElsevier Press.)

Binocular Rivalry

Blake et al, 1990). It is important to note that the results of Lee and Blake do not discredit the existence of

“stimulus rivalry” but, instead, indicate that “stimulus rivalry” represents yet another intriguing form of

perceptual bistability that can arise when normal binocular rivalry is disengaged.

Despite this limitation on the generality of the findings of Logothetis et al (1996), the “stimulus” view of

rivalry is widely accepted in the field of visual neuroscience, having been embraced by a number of investigators

interested in perceptual organization and in conscious awareness. Thus, for example, Andrews and Purves (1997)

opine that “when the stimulus is an ambiguous figure or dichoptically rivalrous, then perception fluctuates between

various possible associations in an ongoing effort to find a definite meaning (p. 9908).” According to this view,

then, binocular rivalry is just one member of a large class of multistable phenomena all involving fluctuations in

alternative object interpretations. Included in that class along with rivalry are Rubin’s vase/face illusion, the Necker

cube, the “old woman/young girl” figure, kinetic depth reversals, the Schroeder staircase and the Hering bent card

illusion. Certainly at the descriptive level, rivalry does behave like the rest of these, with fluctuations in alternative

interpretations over time. And there is some evidence that successive durations of these alternative perceptual states

are independent random variables. However, this resemblance does not necessarily mean that all these phenomena

arise from a common visual mechanism involving the generation of object descriptions. Indeed, there is evidence

(Struber and Stadler 1999) that “top-down” influences (an instruction to “control” the reversal rate) are more

successful for contrast-reversal figures (e.g., the “duck/rabbit” figure) than for perspective-reversal figures (e.g.,

the Necker cube). In addition, the reversal rate tends to increase throughout prolonged viewing of ambiguous

figures (e.g., Brown 1955) while the rate tends to decrease during prolonged viewing of binocular rivalry. The

observation that different multistable phenomena, including binocular rivalry, exhibit comparable temporal

fluctuations may reflect a fundamental property of neural dynamics, but not necessarily a common neural

mechanism.

Besides the influential study by Logothetis et al (1996), there is another strong reason for the growing

popularity of the “stimulus” view of rivalry: it seems to have the force of neurophysiological evidence behind it. In

“higher” temporal lobe areas thought to mediate visual object recognition, all neurons show modulations in

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Binocular Rivalry

activity in synchrony with a monkey’s report of fluctuations in perceptual dominance (Sheinberg and

Logothetis,1997); cell activity, in other words, closely mirrors an animal’s perceptual experience. In contrast,

relatively few neurons in “early” visual areas exhibit such tight coupling between firing rate and perception,

including neurons in area MT (Logothetis and Schall, 1989) and monocular neurons in area V1 (Leopold and

Logothetis, 1996). This pattern of neurophysiological data has been interpreted by some as evidence against “eye”

rivalry, since eye-of-origin information is maintained only within V1 neurons whose activity is mostly unrelated to

fluctuations in dominance. But with all due respect to neurophysiology, data from single-cell recordings cannot

dictate how psychophysical experiments must turn out. A failure to find neurophysiological evidence favoring

“eye” rivalry can not negate perceptual evidence implicating “eye” rivalry; instead, that failure implies that it may

be necessary to rethink the nature of the neural concomitants of rivalry.

Incidentally, the phrase “eye” rivalry is sometimes used synonymously with “early” rivalry, meaning that

the neural events triggering and controlling rivalry transpire at “early” stages of visual processing. It should be

noted, however, that “early” rivalry does not necessarily mean that competition occurs between corresponding

regions of the two eyes. One could imagine an “early” rivalry process that operates on local image features

registered among binocular neurons. Distinguishing “early” from “late” rivalry using psychophysical techniques

is tantamount to determining the extent to which rivalry is governed by cognitive factors, including object

semantics. The brain imaging techniques of functional magnetic resonance imaging (fMRI) and magnetic

encephalography (MEG) can also be employed fruitfully to address this issue of early vs late rivalry (Lumer et al,

1998; Tong et al 1998; Srinivasan et al, 1999; Polansky et al, 2000); elsewhere in this special issue, these

techniques are discussed by Tong (2001).

As the Pendulum Swings

As an historical aside, it is interesting to note that this currently popular view -- stimulus rivalry -- is

reminiscent of a position championed by several noted authorities of their time, including Helmholtz, William

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Binocular Rivalry

James and Charles Sherrington. Although they didn’t use these exact words, they construed rivalry as a high-level,

central process in which fully elaborated monocular views competed for mental dominance. For example,

Sherrington (1906) wrote:

“Only after the sensations initiated from right and left corresponding points have been elaborated, and

have reached a dignity and definiteness well amenable to introspection, does interference between the

reactions of the two eye-systems occur. The binocular sensation attained seems combined from right

and left uniocular sensations elaborated independently.... In retinal rivalry we have an involuntarily

performed analysis of this sensual bicompound. The binocular perception in that case breaks down,

leaving phasic periods of one or other of the component sensations bare to inspection (p. 379).”

This view seems to have survived well into the latter part of this century, as exemplified by Walker’s widely cited

review of the binocular rivalry literature in which he endorses the classical, cognitive view (Walker 1978). It was

Levelt’s influential monograph (Levelt, 1965) that introduced an alternative conceptualization of rivalry, one treating

it as a relatively “low level” process responsive to unrefined image primitives such as luminance contrast and

contour density. Although it took some years for Levelt’s ideas to catch on, by the mid-70’s vision scientists were

beginning to construe rivalry as arising from reciprocal inhibition between feature-detecting neurons in early vision

(Blakemore et al 1972; Wade, 1974 ; Abadi, 1976; Lehky, 1988; Blake, 1989). In the last decade, however, the

pendulum has begun to swing back to the classic, “cognitive” view.

5. On the Neural Bases of Binocular Rivalry

A major purpose of this special issue of Brain and Mind is to highlight binocular rivalry’s potential

usefulness as a “tool” for discovering the neural concomitants of conscious awareness. It is appropriate, therefore,

to conclude this article with some speculations on those concomitants, including some caveats about what to look

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Binocular Rivalry

for and where to look.

5.1 Binocular fusion vs binocular rivalry

What is rivalry’s relation to normal binocular single vision? An enduring idea in the vision literature posits

that rivalry occurs all the time, not just when the two eyes receive dissimilar stimulation (e.g., Asher, 1953).

According to this idea, alternating suppression is the mechanism responsible for single vision; it’s always

occurring but we fail to realize it because the two monocular views are normally identical. Known as suppression

theory, this idea has had adherents as recently as the late twentieth century (eg Wolfe, 1986).

The weight of the evidence now, however, goes against strict suppression theory (e.g., O’Shea 1987),

although binocular suppression may play a role in eliminating diplopia for images formed by objects located far

away from the horopter (Ono et al, 1977). A major reason for believing that binocular single vision is not

attributable solely to suppression is that normal single vision is not accompanied by transient losses in visual

sensitivity that are the hallmark of suppression phases of binocular rivalry (e.g., Wales and Fox 1970). For this

and other reasons, it is now believed that binocular fusion takes precedence over binocular rivalry (Blake and

Boothroyd, 1985). Only when the visual system fails to find left- and right-eye matching features does vision lapse

into reciprocal periods of dominance and suppression. This conclusion is important, for it implies that one should

expect to find the neural concomitants of rivalry only when an observer is explicitly experiencing rivalry and not

when experiencing stable binocular single vision.

5.2 Is rivalry a property of just one of the “multiple” visual pathways?

It is widely accepted that primate vision comprises multiple neural pathways, each specialized to some degree

in terms of its selectivity to spatial and temporal frequency, color, motion and luminance contrast. There remains

active debate concerning the particulars of these pathways and the extent to which they interact, but the general

notion of multiple pathways is deeply ingrained in contemporary visual neuroscience. Is there reason to believe that

the neural events underlying binocular rivalry arise within just one of these putative pathways?

Based on the spatial frequency range yielding the most clear-cut rivalry, Hollins (1980) speculated that rivalry

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Binocular Rivalry

is crucially dependent on activity in what at that time was dubbed the “transient” channel and which today we

would call the “magnocellular pathway.” The same conclusion was reached by Livingstone and Hubel (1987),

who thought that stimulation favoring the parvocellular pathway yielded fusion of dissimilar monocular stimuli, not

rivalry. In subsequent work, however, this conclusion has not received support. For one thing, we know that rival

targets “seen” primarily by the parvocellular pathway can nonetheless yield clear binocular rivalry (Kulikowski,

1992; O’Shea and Williams, 1996). For another, at least some stimulus conditions that should strongly activate the

magnocellular pathway in fact fail to generate binocular rivalry (O’Shea and Blake, 1986; Liu et al, 1992) or yield

weak rivalry at best (Carlson and He, 2000). Certainly, the limited fMRI data on fluctuations in neural activity

during rivalry points to strong modulation in neural activity within ventral stream structures typically associated

with the “form” or parvocellular pathway (Tong et al, 1998).The weight of evidence today is against the

hypothesis that rivalry is a property primarily of the magnocellular pathway.

Is there reason to believe that it is the parvocellular pathway that makes the more robust contribution to

rivalry, as proposed by some (Ramachandran, 1991; Carlson and He, 2000)? As mentioned above, there are good

reasons for believing that rivalry can be experienced under stimulus conditions that activate just the parvocellular

pathway. Moreover, several psychophysical studies indicate that motion information can be integrated between the

two eyes while, at the same time, those eyes are engaged in form rivalry (Andrews and Blakemore, 1999; Carlson

and He, 2000) or in color rivalry (Carney et al, 1987). Recall, too, that motion information associated with a

suppressed rival target can nonetheless generate the translational motion aftereffect (Lehmkuhle and Fox, 1975),

can support perception of apparent motion (Wiesenfelder and Blake, 1991) and can bias perception of otherwise

ambiguous motion (Blake et al, 1998). All these observations are consistent with the hypothesis that rivalry

transpires mainly in the parvocellular pathway, with aspects of vision associated with the magnocellular pathway

escaping rivalry’s vicissitudes. Still, there are studies reporting vigorous binocular rivalry when the two eyes

receive incompatible directions of motion (Blake et al, 1985), including motion at speeds strongly favoring

magnocellular activation (van de Grind et al, in press). Moreover, observers can experience binocular rivalry even

between competing motion aftereffects, provided that those aftereffects are elicited by real motion (Blake et al,

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Binocular Rivalry

1998). It is commonly thought that the MAE arises from neural signals within the magnocellular pathway (e.g.,

Tootell et al, 1995). It is not immediately obvious how to reconcile these latter findings with the idea that rivalry is

the exclusive consequence of parvocellular activity.

5.3 Does rivalry result from distributed neural processes?

Simply observing rivalry, one is fascinated and intrigued that a highly visible, complex, interesting stimulus

can disappear from conscious awareness for several seconds at a time. Though still imaged on the retina, that

stimulus behaves visually as if it intermittently ceased to exist. Based on this phenomenology, one might be

tempted to conclude that the neural events underlying rivalry are as profound as those accompanying intermittent

physical removal of a stimulus. However, this temptation may be misleading -- relatively modest shifts in the

balance of neural activity between different populations of neurons may be sufficient to promote dramatic

fluctuations in the visual appearance of the objects represented by that activity. In this regard, we are reminded that,

during suppression phases of rivalry, visual sensitivity is not dramatically depressed but, instead, is reduced by a

very modest degree. The neural concomitants of suppression, in other words, do not engender a complete blockade

of information transmission but, instead, seem to embody a rather modest gain change. To the extent that this view

is correct, neurophysiologists searching for those neural concomitants may need to look for more subtle changes in

cellular responses coincident with fluctuations in dominance and suppression.

By the same token, psychophysical studies of rivalry may need to rethink how to predict the effect of

suppression on other visual phenomena such as adaptation aftereffects. In much of the past work on this question,

including studies by me, the effects of intermittent suppression during rivalry have been contrasted with intermittent

physical removal of that rival target (e.g., Blake, 1995). But this comparison may be misleading if the neural

concomitants of suppression, in fact, are tantamount merely to attenuating, not eliminating, neural signals associated

with a given stimulus. To illustrate, consider suppression’s failure to reduce the magnitude of the conventional

motion aftereffect (Lehmkuhle and Fox, 1976), which has been interpreted to indicate that registration of

information about translational motion is unaffected by suppression. Perhaps suppression does not block

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Binocular Rivalry

registration of that information, thereby reducing the overall duration of adaptation, but instead dilutes the effective

contrast of the adapting stimulus by an amount insufficient to impact the magnitude of the resulting aftereffect. So,

in brief, we should not necessarily equate phenomenal suppression of a stimulus with physical removal of that

stimulus. To paraphrase an idea advanced by Ramachandran (1991), rivalry need not involve a complete occlusion

of one eye’s signals.

In a related vein, it may also be an oversimplification to speak of rivalry “occurring” at any one particular

neural locus (another oversimplification of which a number of researchers are guilty, including me). Indeed, brain

imaging studies of humans experiencing rivalry (Polansky et al, 2000; Tong et al, 1998; Lumer et al, 1998; Tononi

et al, 1998) and single-unit studies from awake, behaving monkeys experiencing rivalry (Logothetis, 1998) both

imply that traces of the neural signature of rivalry are detectable at early stages of visual processing, with those

traces then being amplified at subsequent, higher stages. Rivalry may result from a cascade of neural events

transpiring at multiple sites along the visual pathways, with no single site constituting “the” locus of rivalry (e.g.,

Ooi and He, 1999).

There is another sense in which rivalry could result from distributed processes, not a single mechanism

operating at one level of vision. As noted above, binocular rivalry can be characterized in terms of its spatial extent,

its temporal dynamics and its generality beyond those conditions triggering rivalry. These aspects of rivalry are not

necessarily tied to a single, omnibus process, although one frequently encounters the phrase “rivalry mechanism”

in discussions of binocular rivalry. The stimulus determinants of fluctuations in dominance and suppression (i.e.

temporal dynamics) are not necessarily those governing the spatial extent of rivalry. Similarly, it is conceivable that

the well-established stimulus determinants of suppression phases (i.e., energic variables like luminance and

contrast) differ from those controlling dominance durations (e.g., context, meaning). Indeed, to reconcile the

diverse findings concerning rivalry - including controversial results on the role of meaning and context in rivalry -

it may be important to distinguish between processes responsible for initiation of rivalry and selection of one eye’s

input for dominance from processes responsible for the implementation and maintenance of suppression. It is

entirely plausible that suppression and selection are the result of separate mechanisms operating at different stages

27

Binocular Rivalry

in the visual system. In a sense, selection involves the comparison of information presented to the two eyes to

establish the degree of correspondence and, perhaps, the relation of a given stimulus to other features within the

visual field. Once selection has been accomplished, one stimulus (or portions of it) is temporarily “rejected” (i.e.

suppressed) while another stimulus dominates. The fate of the suppressed stimulus - and other, new information

presented within its immediate vicinity - can be determined by neural events different from those registering the

initial incompatibility of monocular stimuli. This idea of rivalry being a dual process has been elaborated by Fox

(1991), and it offers a potentially useful scheme for reconciling seemingly conflicting results within the rivalry

literature.

6. Concluding Remarks

To close on a personal note, twenty six years ago when I published my dissertation work on binocular rivalry

(Blake and Fox, 1974a, 1974b), the phenomenon was considered by some to be a laboratory curiosity of

questionable relevance for an understanding of perception.5 Since that time, however, binocular rivalry has been

“rediscovered” by the neuroscience community, undoubtedly because of its potential as a tool for studying the

neural concomitants of visual awareness using both single cell responses (Logothetis, 1998; Engel et al, 1999) and

brain imaging techniques (Tong et al, 1998; Polansky et al, 2000). Accompanying rivalry’s rediscovery has been a

flurry of clever psychophysical studies of rivalry that, among other things, challenge models positing that rivalry is

based on competing eyes (Blake, 1989) and strengthen the case for the involvement of context, attention and

meaning in the promotion of dominance. For seasoned aficionados of binocular rivalry, it is gratifying, not to

mention invigorating, to experience a resurgence of interest in this fascinating and still enigmatic phenomenon.

5. J.J. Gibson (1966) colorfully characterized rivalry as “optical trickery.”

28

Binocular Rivalry

Acknowledgements

The author is supported by a research grant (EY07760) and a core grant (EY01826) from the National Institutes of

Health. I am grateful to Ilona Kovács, Thomas Papathomas, Tania Czarneck, Kenneth Sobel, Robert O’Shea and

Sang-Hun Lee for comments on earlier drafts of this article, and to Sang-Hun Lee and Jennifer Jewett for help

creating some of the figures. This article is dedicated to my mentor, colleague and friend Robert Fox who first

introduced me to binocular rivalry. His enthusiasm and creativity sparked my interest in this intriguing

phenomenon, and his sage advice kept me pointed in potentially fruitful directions. While he will not agree with all

the ideas advanced in this paper, Robert Fox will recognize his imprint on these pages.

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